class Stack:
    def __init__(self, start=[]):  #初始化栈
        self.stack = []
        for x in start:
            self.push(x)

    def isEmpty(self):
        return not self.stack

    def push(self, obj):
        self.stack.append(obj)

    def pop(self):
        if not self.stack:
            print("栈为空！")
        else:
            return self.stack.pop()
    def top(self):
        if not self.stack:
            print("栈为空！")
        else:
            return self.stack[-1]

    def bottom(self):
        if not self.stack:
            print("栈为空！")
        else:
            return self.stack[0]
